Scatter radiation is one of the most serious problems in radiography. It reduces subject contrast to as little as 10% of its intrinsic value and requires the use of high contrast x-ray photographic films with their concomitant exacting exposure and processing requirements.
Various methods currently exist to remove, or reduce, this scatter radiation. The most common is a mechanical system which "collimates" or reduces the acceptance angle of the detector to the scatter radiation. Conventional devices of this type (such as the slat grid, moving grids, or rotating apertures) are rather heavy. A grid, in fact, is often not used because it is too heavy to carry to the bedside for portable radiography. Conventional slat grids are made by alternating strips of lead foil with strips of aluminum or fiber. See U.S. Pat. No. 1,476,048 to Gustov Bucky issued Dec. 4, 1923. The aluminum or fiber "interspace material" is required to keep the lead foils separated and aligned. In addition to being heavy and fragile, fiber interspace grids are susceptible to humidity problems. Neither type (aluminum or fiber) can be repaired should they be accidentally dropped, and both types increase patient exposure due to the absorption of primary radiation by the interspace material.
A greatly enlarged cross sectional portion of a simple, conventional grid is schematically shown in FIG. 2. In the grid, x-ray opaque lead foil slats 10 alternate with filler strips 12 such as aluminum or fiber. The height of the grid is h, and the interspace width is d. The ratio r=h/d is known as the grid ratio. In practice this ratio h/d=16/1 is considered maximum. To achieve this ratio without reducing the transmission of the grid requires a large number of slats (i.e., a small value of d), since the available h is limited by the current use and design of x-ray equipment to values of about two millimeters.
The required large number of slats results in a grid that is very heavy. It is therefore an object of the present invention to provide a method and a grid for medical radiography that is lighter in weight than conventional grids.
Another type of grid, shown in U.S. Pat. No. 2,605,427 issued July 29, 1952 to Delhumeau is a two-dimensional focusing grid, so called because the slats are aligned with the rays coming from the x-ray source. Two-dimensional grids are nearly twice as heavy as one-dimensional grids due to the extra x-ray absorbent material.
It is therefore a further object of the invention to provide novel light weight two-dimensional grids and in particular, two-dimensional focusing grids.
In the prior art practice of bedside radiography where an x-ray cassette is slipped under a critically ill patient and an x-ray exposure is performed at the patient's bedside, grids were frequently not employed due to their bulk and difficulty of handling. The resulting exposures suffered due to scatter. Therefore, it is a still further object to provide a lightweight grid that can be incorporated into a standard x-ray cassette.